Microwave electron discharge device having annular resonant cavity



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United States Patent 3,219,873 MICROWAVE ELECTRON DISCHARGE DEVECEHAVING ANNULAR RESGNANT CAVITY Irving Kaufman, Woodland Hills, Calif.,assignor, by mesne assignments, to TRW Inc., a corporation of Ohio FiledSept. 1, 1961, Ser. No. 135,640 1 Claim. (Cl. 3155.25)

The present invention relates generally to the art of generating,amplifying, and frequency multiplying microwave signals, and isespecially adapted and useful for converting direct current power toradio frequency power in the millimeter and submillimeter wavelengthregions. More particularly, this invention has reference to frequencymultipliers, oscillators, and amplifiers employing microwave cavityresonators excited by controlled electron beams.

It is a general object of the present invention to provide improvedapparatus for efiiciently producing electric waves in the millimeter andsub-millimeter wavelengths.

It is a primary object to overcome the high frequency limitations ofknown microwave generators and amplifiers by providing apparatus whichis inherently independent of certain basic handicaps of the commonlyused prior art techniques and devices.

It is a further object to facilitate the manufacture and reduce the costand complexity of microwave active systems.

Various devices are known for generating radio frequency power atwavelengths of a few millimeters. Such devices which have utilizedlinear electron beams are divisible operationally into two generalclasses: (1) devices employing longitudinal compression bunching of theelectron beam, and (2) devices using radio frequency deflection of anelectron beam in conjunction with a slotted or apertured target forchopping the beam.

In systems of the first type above mentioned, such as klystrons andtraveling wave tubes employing beam density modulation or beam velocitymodulation, the electron bunches created necessarily must be shorterthan the wavelength to be generated. It has been diflicult to fulfillthat requirement with appreciable electron densities at millimeterwavelengths. Extremely low efliciencies have resulted. Moreover, even ifsufficiently short electron bunches are created in such devices, thebunches are quickly distended or stretched by longitudinal space chargeforces within the beam.

Referring to the second class of devices mentioned above, it is a verysimple matter to produce electron bunches by the beam choppingtechnique. However, such systems have the extreme disadvantage that, asthe frequency is increased, the electron bunch is shortened withoutbeing increased in density and therefore contains proportionally fewerelectrons. Accordingly, the amount of microwave output power which canbe produced from the reduced energy bunches becomes prohibtively smallas the frequency is increased.

The present invention overcomes the foregoing handicaps of previoussystems by continuously utilizing substantially all of the beam currentproduced by the electron gun. In addition, the present inventionincreases the useful kinetic energy of the beam electrons withoutincreasing the load imposed on the con-trolling microwave input signalsource. Even more important, the present invention reduces, by an orderof magnitude, the deleterious effects of aberrations in the longitudinalvelocity of the cathode ray beam electrons.

The foregoing desirable effects are accomplished by using a cathode raygenerating system in which the electron beam is circularly orelliptically deflected under the control of and in synchronism with aninput micro- 3,219,873 Patented Nov. 23, 1965 wave signal. The beamthereby describes a conical helix, with the electrons of the beamrespectively traversing successive portions of a microwave resonatordevice which is adapted to derive energy from the electron beam. In apreferred embodiment of the invention, the microwave resonator comprisesan annular waveguide member positioned in a plane substantiallyperpendicular to the axis of the conical beam and arranged to internallyreceive the electrons of the beam.

The microwave resonator is constructed and arranged to provide adecelerating electric field in the region where each electron isinternally traversing the structure. By decelerating the beam electrons,the electromagnetic Waves Within the microwave resonator absorb kineticenergy from the rapidly moving electrons and thereby regenerate themicrowave oscillations within the resonator. Further, in accordance withpreferred embodiments of the invention, the electron beam is conicallydeflected by the input microwave signal at a point in the system wherethe beam electrons have a relatively low velocity. The conical beam isthereafter increased in energy by a direct current energizedaccelerating system so that the total microwave energy which may bederived from the conical beam is finally several orders of magnitudegreater than the microwave input power required for deflecting the beam.

The present invention, together with further objects and advantagesthereof, may be best understood by reference to the followingdescription taken in accordance with the accompanying drawings, inwhich'like reference characters indicate like parts and in which:

FIGURE 1 is a schematic illustration of a microwave power conversionsystem of the general type to which the invention relates;

FIGS. 2a and 2b are respectively a front View and a side view of theannular microwave resonator used in the system of FIG. 1;

FIG. 3 is a fragmentary sectional view representing a structuralmodification of the embodiment of 'FIG. 1 and FIG. 4 is a schematicdiagram of a further embodiment in accordance with the invention.

Referring now to FIG. 1, numeral 12 designates a cathode ray producingelectron gun including a cathode 14- and a beam forming acceleratingelectrode .16. The electron beam generating means or electron gun 12 maytake any one of various forms well known in the art. The onlyrequirements of the present invention are that the electron gun 12should produce a Well defined pencil beam of electrons which isinitially projected along the horizontal axis 15. Along the electronbeam axis 15 immediately subsequent to the beam forming electrode 16 ispositioned a deflection system 18 comprising a pair of Lecher-wires orresonant transmission line sections 20 and 22.

As shown in FIG. 1, the Lecher-wire pair 20 is disposed in a planetransverse to the beam axis and extending perpendicularly to the planeof the paper. The second Lecher-wire pair 22 is disposed in a planesubstantially parallel to that of the first Lecher-wire pair, but withthe second pair 22 at right angles to the first pair. Thus, thedeflection system comprises two crossed pairs of shorted Lecher-wires,preferably resonant at the frequency of the microwave input signal. Theintersection of the two line pairs 20 and 22 is preferably locatedone-quarter wavelength away from' the shorted ends of the line pairs sothat voltage maxima occurring on the lines 2t) and 22 will be located atthe center of the small square window which is framed by the two wirepairs. Thus, the electron beam which passes through the center of thewindow is subjected to the maxi-mum cables 24 and 26 which arerespectively connected to the .first and second line pairs 20. and 22.The input signal "is applied directly to the transmission line 24 fromthe signal source 30. The input signal is applied from the source 30through a phase shifting device 28 and thence to the line 26. The phaseshifting device 28, which may be of conventional construction, providesapproximately a 90 phase differential between the electromagnetic wavesat the transmission lines 24 and 26. It will be appreciated that thebeam transit time between the planes of lines 24 and 26 must be takeninto account in providing the cptimum phase differential between the twosignals; that is, if the phase differential were exactly 90, thedeflection effect obtained would be different from that which is desiredby the amount of the beam transit time. Thus, in actual practice, aphase differential slightly exceeding 90 is to be desired. The phasedifference between the signals applied to the line pair 20 and the linepair 22 causes the deflection system to circularly deflect the electronbeam. Thus the electron beam, after passing through the deflectionsystem 18, describes :a conical rotating trace. More exactly, since theperiod of the input microwave signal is considerably shorter than thetransit time of the beam electrons between the deflector and resonator,the

beam electrons are distributed in a helical array along the surface ofan imaginary right circular cone extending from the deflection system 18to the microwave resonator device 36.

The deflection system 18 comprising the crossed Lecherwires 20 and 22may be constnucted generally in accordance with the teachings of anarticle entitled UHF Beam Analyzer, by L. R. Bloom and H. M. VonFoester, Review of Scientific Instruments, vol. 25, July 1954, pages649-653. Complete information as to optimum designs of such microwavedeflection systems is there provided and is incorporated herein byreference.

Referring to FIGS. 2a and 2b, the microwave resonator 36 preferablycomprises a length of rectangular waveguide, conventionally designed ofhighly conductive material, and bent into a circular or ellipticalshape, with the ends joined together to form a toroid. Thecross-sectional di mensions of the waveguide are chosen in accordancewith conventional theory to make the device resonant at a frequencycorresponding to an integral multiple of the frequency of the inputsignal provided by source 30. The mean circumferential length of theannular resonator 36 is preferably made equal to an integral number ofwave guide wavelengths at the harmonic frequency which is to beproduced. An output signal transmission line 40, comprising a waveguidesection conductively attached to one peripheral portion of the annularresonator 36, enables extraction of microwave power at the harmonicfrequency for utilization in any desired load system. Microwave energycommunication from the annular resonator 36 to the output coupler 40 isenabled by an aperture communicating interiorly between the two. Theoutput signal coupling from resonator 36 to coupler 40 may take any oneof various known forms, such as, for example, a crossed-slot coupling.

The annular resonator 36 may be designed to have either the usualstanding wave excitation or a traveling wave moving circularly aroundthe resonator. In order to minimize the transit time of electronstraversing the waveguide resonator from front to back, the resonator 36is preferably provided with a reentrant portion 38 providing an interiorridge or shoulder in the fashion of waveguide structures of theso-called ridged type. The details of structure of the resonator 36 arenot belabored here, since the same are well known to those skilled inthe art. For example, the dimensional details of the annular waveguideresonator 36 of the present invention may be generally in accordancewith the teachings of an article entitled Resonance Properties of RingCircuits, by F. J. Tischer, IRE Transactions on Microwave Theory andTechniques, vol. MTT-S, January 1957, pages 5156.

The one essential departure from conventional structure whichcharacterizes the annular resonator of the present invention is that theresonator 36 is circularly slotted in the side facing the electron gunand along a line substantially coinciding with the mean radius of theannulus and exactly coinciding with the circular intersection of theconical electron beam and the front surface of the resonator. The backsurface of the waveguide may be similarly slotted along the path of thetraversing electrons, or, more exactly, the inner portion of thereentrant ridge 38 is so slotted. Provision of the circular slots orapertures in the waveguide resonator permits the high velocity electronsto travel through the resonator and subsequently impinge upon a targetor collector, thereby avoiding the generation of deleterious secondaryelectrons within the resonator itself.

If the electrons, after traversing the resonator 36, were allowed tostrike a target or anode at the same direct current potential as theresonator, a considerable amount of heat loss would occur at thecollector. Accordingly, in the preferred embodiment a Faraday cupcollector 42 is provided. The Faraday collector 42 may be a circularhollow cup-like structure having rear and front walls 43 and 44,respectively, with the front wall 44 circularly slotted as at 45, withthe circular slot being coincident with the conical line of flight ofthe electron beam so that the electrons are collected interiorly of theFaraday collector 42 to avoid generation of secondary electrons.

For simplicity in the embodiment shown schematically in FIG. 1, theaccelerating electrode 16 is shown as being grounded at 17, and themicrowave resonator 36 is grounded at 37. With this arrangement, thecathode 14 is preferably connected to a direct current potenetial sourceof 20 kv. negative with respect to ground. To minimize collector heatloss, the Faraday collector cup 42 is operated at a potential onlyslightly positive with respect to the cathode potential and therefore ata potential of about -19 kv. with respect to ground.

Interposed in the conical beam drift space between the deflection system18 and the microwave resonator 36, there is provided an acceleratinganode 29 comprising inner and outer concentric rings which may serve aselectro-optical lenses to accelerate the conical beam and to concentratethe beam electrons on the circular slot in the front Wall of theresonator 36. Accelerating anode 29 is preferably operated at apotential which is substantially positive with respect to the deflectionsystem 18 and the first accelerating electrode 16.

To clarify the operation of the system of FIG. 1, it is advantageous tofirst consider that the resonator 36 supports a travelingelectromagnetic wave at the harmonic frequency which is to be generated.It may be seen that if the waveguide wavelength in the resonator 36 is Athen any wave traveling around the resonator will arrive at the startingpoint in phase with another wave starting from that same point. Thus,the physical requirement for a distributed resonator is fulfilled if themean circumferential length of the resonator 36 is an integral multipleof A An electron which traverses the resonator 36 from the front wall tothe back and in a retarding electric fieldi will deliver energy to thatfield. The motion of the peak of the electron retarding phase of theradio frequency electric field within the resonator 36 is circular atthe phase velocity of the harmonic traveling wave, and the electricfield is perpendicular to the plane of the resonator annulus andtherefore substantially parallel to the path of the electrons traversingthe resonator. By causing electron beam to circularly scan the resonatorat a phase velocity coinciding with the phase velocity of the travelingwave Within the resonator, the individual electrons traverse successiveperipheral portions of the resonator in synchronism with the existenceof the peak electron retarding electric field in the successive portionsof the resonator. Thus, the beam electrons successively and continuouslydeliver energy to the electric field within the resonator. In thismanner, electromagnetic oscillations are built up within the resonatoruntil a level is reached at which the average power derived from theelectron beam equals that delivered to the output load plus theresistive power losses in the resonator walls. Since the deflectionsystem 18 produces a circular or rotating electron beam at the frequency of the input signal from source 30, the effective linear sweep atthe resonator 36 is:

Because the frequencies and the dimensions of the resonator 36 have beenchosen so that the resonator 36 supports a harmonic of the inputfrequency f, the circular phase velocity of the electron beam is insynchronism with the radio frequency traveling wave inside the resonator36. Accordingly, the frequency of the wave generated in the resonator 36is:

Accordingly, the output microwave signals produced by resonator 36 andextracted through coupler 40 will be the nth harmonic of the microwaveinput signal provided by source 30.

It is important to note that it is not necessary for the circular phasevelocity of the scanning beam to be less than the velocity of light.Each electron of the beam moves in a straight line along a polar elementof the cone at a speed determined by the electron gun acceleratingpotential and the deflection system. The electron velocities areobviously less than the velocity of light. The circular sweep, however,rotates at the frequency of the input signal source 30 so that theeflective linear scan speed of the beam at the cone surface increaseswith the distance from the deflection system 18. The linear scan speedof the beam at the resonator 36 is thus a phase velocity, not an actualelectron velocity, and therefore the scan speed can approach and evenexceed the speed of light. This is an extremely important considerationbecause it permits the resonator 36 to be a hollow metal waveguidestructure not loaded by dielectric or other slow wave materials andhaving electromagnetic fields which extend throughout the interiorvolume of the resonator.

In FIG. 3 is shown an alternative embodiment or modification of the beamgeneration system, the beam deflection system, and accelerating systemof the apparatus in accordance with the invention. The electron gun 12,cathode 14, and collimating electrode 16 of FIG. 3 are structurallyidentical to the same elements of FIG. 1, and accordingly are indicatedby the same reference numerals. Likewise, the deflection system is shownin FIG. 3 as comprising a pair of crossed Lecher-wires 20 and 22identical to the same elements of FIG. 1. The system of FIG. 3 differsfrom that of FIG. 1 in that it includes a postdeflection beamacceleration system comprised of first and second semisphericalelectrodes 32 and 34. The electrodes 32 and 34 are circularly slottedalong the conical beam path to permit the conical beam to projectoutwardly to the right beyond the electrode 34. In the apparatus of FIG.3, the collimating electrode 16 is connected to a point of l9 kv. withrespect to ground. Similarly, the

cathode 14 is connected to 20 kv. The first semispherical lens electrode32 is connected commonly with collimating electrode 16 to the l9 kv.source, and the second semispherical lens electrode 34 is connected toground or the point of reference potential. The two shorted Lecher-wiredeflection elements 20 and 22 are connected together by a first radiofrequency choke 35 and are connected to the collimating electrode 16 bymeans of a second radio frequency choke 33. It is to be noted that inthe arrangement of FIG. 3 the beam electrons, upon arrival at thedeflection system, have been accelerated by only 1000 volts, i.e., thepotential between the cathode 14 and the collimating electrode 16. Thusthe electron beam, as it traverses the deflection system 20 and 22, is arelatively soft and easily deflected beam which requires verysubstantially less deflection system input energy than would be requiredby the corresponding deflection system of FIG. 1.

The first semispherical electrode 32, being connected to the samepotential as the collimating electrode 16, shields the deflection systemfrom high potential gradients and thereby minimizes deflectiondistortion which might otherwise occur. Between the two semisphericalelectrodes 32 and 34 there exists an intense potential gradient ofapproximately 19 kv. This potential gradient operates to accelerate theconical deflected beam so as to increase the kinetic energy of each beamelectron by several orders of magnitude. It is emphasized here that thefinal kinetic energy of the conical beam, as it emerges from theelectrode 34, is provided primarily by the direct current voltage sourceand not by the microwave energy fed to the deflection system. As statedheretofore, this is an extremely important consideration because of thelow powers available from practical sources of microwave input power.

For simplicity in describing the systems shown in FIGS. 1 and 3, thedeflection system has been set forth as comprising a pair of shortedLecher-wires disposed at right angles to provide a small square windowthrough which the electron beam is projected. The use of Lecher-wiresfor deflection is not intended to be an essential characteristic of thepresent invention. The Lecher-wire system has been set forth by way ofexample as one completely satisfactory and operative arrangement. Othersystems, such as a Waveguide cavity or a ridged waveguide cavity, orparallel plate lines, would be acceptable as a deflection means in thesystem of the present invention and are considered to be within thebroad scope of the invention. In all of the above-mentioned alternativedeflection systems, each electron leaving the deflection system wouldtravel in a straight line along an element of the cone, therebysatisfying the requirements for properly exciting the annu lar microwaveresonator 36. The above-mentioned alternative deflection systems may beadvantageous as compared to the Lecher-wire system in that they areselfshielding systems, whereas the Lecher-wires are open radiators andwould expend a considerable amount of input deflection power byradiation to the surroundings, with only a relatively small portion ofthe power-being used for actual deflection of the electron beam. Oneadvantage of the Lecher-wire pair deflection system in that it can befitted into a relatively small space as compared to waveguide cavitieswhich might be used for deflection.

In the embodiments set forth in FIGS. 1 and 3, the deflection systemsare followed by a long drift distance necessary to provide a beam conediameter corresponding to that of the annular resonator 36. A relativelylong drift distance in the embodiment of FIG. 1 is necessary to minimizethe deflection angle and the deflection input power. However, the use ofa long drift distance is disadvantageous in that (1) it limits operationof the system to relatively low beam currents and therefore relativelylow magnitudes of harmonic power output; and (2) with long driftdistance, aberrations in longitudinal electron velocity introduced bythe deflection system produce an undesirably large spread in the arrivaltime of the electrons at the annular resonator. That is true becauseaberrations in longitudinal velocity produce uncertainties in arrivaltime which are directly proportional to the transit time as determinedby the drift distance. Since each electron must arrive at a particularangular portion of the resonator simultaneously with the arrival of thepeak of the electric field at that portion of the resonator, the spreadin arrival time caused by long drift distance tends to limit the maximumfrequency (minimum wavelength) for which the annular resonator may bedesigned. Furt-her, aberrations in the deflection system may distort thecircular sweep into a slightly elliptical sweep. Because of thenecessarily small width of the circular slot in the front wall of theresonator 36, such ellipticity is undesirable. The dimensionalellipticity, of course, can be minimized by reducing the beam driftdistance.

FIG. 4 illustrates a further embodiment in accordance with the presentinvention which avoids the foregoing possible disadvantages of theearlier embodiments by providing for an extremely short drift distance.As shown in FIG. 4, the beam forming electron gun 12 may be identical tothe electron gun of FIG. 1, and the deflection system 18 comprisingLecher-wires 20 and 22 is similar in structure and function to the sameelements of FIG. 1. The apparatus of FIG. 4 differs from the precedingembodiments in the structure of the annular waveguide resonator and inthe structure and arrangement of the postdeflection acceleration system.The waveguide resonator 48, as shown in FIG. 4, is generally cylindricalin shape and is designed to support an annularly propagatingelectromagnetic wave at the desired harmonic frequency, with theelectric field of the wave extending radially within the annulus andtherefore at right angles to the axis of symmetry of the system. Thewaveguide annulus 48 is shown as being a ridged waveguide, with theinternal ridge being formed by a reentrant portion extending inwardlyfrom the outer diameter of the cylinder.

Since the electric field extends radially within the waveguide annulus,it is necessary that the electron beam traverse the annulus more or lessradially from the inner diameter to the outer diameter of the ridge. Tothat end there is provided within the annular waveguide resonator 48 anelectrostatic post-deflection acceleration system and beam bendingsystem. This system is comprised of a cylindrical conductive rod 54disposed on the axis of symmetry of the system and connected to apositive potential of about 1 kv., which is the same as the potential ofthe collimating electrode 16 in the electrode gun 12. The

annular waveguide resonator 48 is connected to a point of about 20 kv.positive with respect to the cathode, thereby providing an intenseelectrostatic field between the rod 54 and the resonator 48. Thisintense electrostatic field acts on the slightly deflected beam to bendit outwardly and project it more or less radially through the slot 50 ofthe annular resonator 48. It will be seen that this arrangement shortensthe effective drift distance of the beam by as much as an order ofmagnitude. Since the final direction of electron motion here is nearlyperpendicular to the axis of symmetry of the system, the electronstraverse the annular resonator radially and therefore in the properdirection for energy transfer interaction with the retarding electricfield within the resonator.

In the arrangement of FIG. 4, the radio frequency deflection system 18produces a consecutive time circular sweep identical to that produced bythe deflection system of FIG. 1. However, the circular sweep in theapparatus of FIG. 4 not only provides a slight angular deflection of thebeam, but it also selects the proper azimuth or proper radial directionfor each successive electron. Since each segment of the beam moves alonga ray of the same polar angle, its path may be bent outwardly by theaxially symmetrical electrostatic field without changing the radialdirection of the beam segment. The important consideration in the systemof FIG. 4 is that the beam bending, and hence most of the radialvelocity of the electrons, is provided by energy from the direct currentvoltage source so that the fundamental frequency power which must be fedinto the deflection system 18 is minimized. In addition, it is importantto note that the time aberrations, which become serious when long driftdistances are employed, are avoided, thereby making it possible to use aconsiderably narrower slot 50 in the inner and outer walls of theresonator 48 and making it possible to design the resonator 48 foroperation at shorter wavelengths than would be possible for the systemof FIG. 1.

As shown in FIG. 4, the Faraday cup collector 52 takes the form of ahollow annulus which is slotted on its inner diameter to receive thebeam electrons after they exit from the resonator 48.

A basic advantage of the system in accordance with FIG. 4 is that theelectron beam, as it traverses the deflection system 18, is a relativelylow velocity beam which requires much less power for a given angulardeflection than would be required in the system of FIG. 1. Furthermore,because of the beam bending arrangement employed in the apparatus ofFIG. 4, the angle of deflection which must be provided by the deflectionsystem 18 is very small, thereby further decreasing the load imposed onthe source 30 of microwave fundamental frequency energy which suppliesthe deflection system 18. Beam acceleration is provided primarily by theelectric field between elements 48 and 54. The deflection system isrequired to provide only a relatively small amount of transversevelocity to the beam electrons. Most of the radial velocity of theelectrons is furnished by the direct current voltage source, and not bythe deflection system. Reducing the deflection system power consumptionis important because of the low power output levels which are availablefrom practical microwave sources. If a reasonably practical amount ofharmonic frequency power were to be extracted from a system such as thatof FIG. 1, but not employing post-deflection acceleration, it would benecessary to supply fundamental frequency input power to the deflectionsystem in amounts approaching 1 watt. That much input power atmillimeter and submillimeter wavelengths is not readily and economicallyavailable. The present invention overcomes that difficulty by furnishingmost of the radial kinetic energy of the beam electrons, not from theradio frequency deflection system, but rather from the direct currentenergizing source as applied to the post-deflection accelerating means.

In addition to reducing the required radio frequency input power,post-deflection acceleration also proportionally reduces the amount ofaberration introduced by the deflection system. Reduction of suchaberration is of great value because it reduces the spread in electronarrival times at a given angular point on the output resonator andthereby enables use of the system at higher frequencies. In addition, ashorter drift distance has the advantage that for a given resonator slotwidth appreciably greater beam currents may be used. The output poweris, of course, increased by increased beam current.

While there have been described what are at present considered to bepreferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is aimed in theappended claim to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivelege is claimed are defined as follows:

In a microwave frequency converter comprising a discharge device havingbeam forming means for providing an axially directed beam of chargedparticles:

deflection means for imparting conical rotation to said beam in responseto deflection signals of a first frequency;

a generally cylindrical microwave resonator positioned in symmetricaland axial alignment with the central axis of said conically rotatingbeam;

said resonator comprising a hollow annulus of conductive materialinternally dimensioned to support annularly traveling electromagneticwaves of a second frequency having their electric field vectorsextending substantially radially with respect to said axis;

means for positively biasing said annulus relative to said beam formingmeans;

and post-deflection acceleration means, comprising a substantiallycylindrical dynode member having a relatively negative direct currentpotential, positioned symmetrically with respect to said beam axisadjacent the lines of flight of said charged particles for acceleratingand outwardly deflecting said particles so that the same traverse saidannulus in a direction to impart kinetic energy to the second frequencyelectric fields within said annulus.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESResonance Properties of Ring Circuits; article by F. J.

Tischer, pages 51-56, IRE Transactions on Microwave Theory andTechniques, vol. MTT-5, January 1957.

UHF Beam Analyzer; article by L. R. Bloom and 15 H. M. Foester, pages649653, Review of Scientific Instruments, vol. 25, July 1954.

HERMAN KARL SAALBACH, Primary Examiner.

20 BENNETT G. MILLER, GEORGE N. WESTBY,

Examiners.

